Enabling estimation of an initial point of interaction of an x-ray photon in a photon-counting x-ray detector

ABSTRACT

The disclosed method enables estimating an initial point of interaction of an x-ray photon in a photon-counting x-ray detector, based on a number of x-ray detector sub-modules or wafers, each including detector elements. The x-ray detector sub-modules are oriented in edge-on geometry with the edge directed towards the x-ray source, assuming the x-rays enter through the edge. Each detector sub-module or wafer has a thickness with two opposite sides of different potentials to enable charge drift towards the side, where the detector elements, also referred to as pixels, are arranged. Basically, the method includes: determining an estimate of charge diffusion originating from a Compton interaction or an interaction through photoeffect related to the x-ray photon in a detector sub-module or wafer of the x-ray detector; and estimating the initial point of interaction along the thickness of the detector sub-module based on the determined estimate of charge diffusion.

TECHNICAL FIELD

The proposed technology relates to x-ray imaging and x-ray detectors,and more particularly to a method and corresponding system for enablingestimation of an initial point of interaction of an x-ray photon in aphoton-counting x-ray detector, as well as a corresponding x-ray imagingsystem, computer program and computer-program product.

BACKGROUND

Radiographic imaging such as x-ray imaging has been used for years inmedical applications and for non-destructive testing.

Normally, an x-ray imaging system includes an x-ray source and an x-raydetector system. The x-ray source emits x-rays, which pass through asubject or object to be imaged and are then registered by the x-raydetector system. Since some materials absorb a larger fraction of thex-rays than others, an image is formed of the subject or object.

It may be useful to begin with a brief overview of an illustrativeoverall x-ray imaging system, with reference to FIG. 1. In thisnon-limiting example, the x-ray imaging system 100 basically comprisesan x-ray source 10, an x-ray detector system 20 and an associated imageprocessing device 30. In general, the x-ray detector system 20 isconfigured for registering radiation from the x-ray source 10 that mayhave been focused by optional x-ray optics and passed an object orsubject or part thereof. The x-ray detector system 20 is connectable tothe image processing device 30 via suitable analog processing andread-out electronics (which may be integrated in the x-ray detectorsystem 20) to enable image processing and/or image reconstruction by theimage processing device 30.

FIG. 2 is a schematic diagram illustrating an example of an x-rayimaging system 100 comprises an x-ray source 10, which emits x-rays; anx-ray detector system 20, which detects the x-rays after they havepassed through the object; analog processing circuitry 25, whichprocesses the raw electrical signal from the detector and digitizes it;digital processing circuitry 40 which may carry out further processingoperations on the measured data such as applying corrections, storing ittemporarily, or filtering; and a computer 50 which stores the processeddata and may perform further post-processing and/or imagereconstruction.

The overall detector may be regarded as the x-ray detector system 20, orthe x-ray detector system 20 combined with the associated analogprocessing circuitry 25.

The digital part including the digital processing circuitry 40 and/orthe computer 50 may be regarded as a digital image processing system 30,which performs image reconstruction based on the image data from thex-ray detector. The image processing system 30 may thus be seen as thecomputer 50, or alternatively the combined system of the digitalprocessing circuitry 40 and the computer 50, or possibly the digitalprocessing circuitry 40 by itself if the digital processing circuitry isfurther specialized also for image processing and/or reconstruction.

An example of a commonly used x-ray imaging system is a ComputedTomography (CT) system, which may include an x-ray source that producesa fan or cone beam of x-rays and an opposing x-ray detector system forregistering the fraction of x-rays that are transmitted through apatient or object. The x-ray source and detector system are normallymounted in a gantry that rotates around the imaged object.

Accordingly, the x-ray source 10 and the x-ray detector system 20illustrated in FIG. 1 and FIG. 2 may thus be arranged as part of a CTsystem, e.g. mountable in a CT gantry.

A challenge for x-ray imaging detectors is to extract maximuminformation from the detected x-rays to provide input to an image of anobject or subject where the object or subject is depicted in terms ofdensity, composition and structure. It is still common to usefilm-screen as detector but most commonly the detectors today provide adigital image.

Modern x-ray detectors normally need to convert the incident x-rays intoelectrons, this typically takes place through photo absorption orthrough Compton interaction and the resulting electrons are usuallycreating secondary visible light until its energy is lost and this lightis in turn detected by a photo-sensitive material. There are alsodetectors, which are based on semiconductors and in this case theelectrons created by the x-ray are creating electric charge in terms ofelectron-hole pairs which are collected through an applied electricfield.

Conventional x-ray detectors are energy integrating, the contributionfrom each detected photon to the detected signal is thereforeproportional to its energy, and in conventional CT, measurements areacquired for a single energy distribution. The images produced by aconventional CT system therefore have a certain look, where differenttissues and materials show typical values in certain ranges.

There are detectors operating in an integrating mode in the sense thatthey provide an integrated signal from a multitude of x-rays and thesignal is only later digitized to retrieve a best guess of the number ofincident x-rays in a pixel.

Photon counting detectors have also emerged as a feasible alternative insome applications; currently those detectors are commercially availablemainly in mammography. The photon counting detectors have an advantagesince in principle the energy for each x-ray can be measured whichyields additional information about the composition of the object. Thisinformation can be used to increase the image quality and/or to decreasethe radiation dose.

A further improvement relates to the development of so-calledenergy-discriminating photon-counting detectors, e.g. as schematicallyillustrated in FIG. 3. In this type of x-ray detectors, each registeredphoton generates a current pulse which is compared to a set ofthresholds, thereby counting the number of photons incident in each of anumber of so-called energy bins. This may be very useful in the imagereconstruction process.

WO 2017/015473 relates to detector designs and systems for enhancedradiographic imaging with integrated detector systems that incorporateone or more of Compton and nuclear medicine imaging, PET imaging andx-ray CT imaging capabilities. Detector designs employ one or morelayers of detector modules comprised of edge-on or face-on detectors ora combination of edge-on and face-on detectors which may employ gas,scintillator, semiconductor, low temperature (such as Ge andsuperconductor) and structured detectors. Detectors may implementtracking capabilities and may operate in a non-coincidence orcoincidence detection mode.

US 2011/0155918 relates to systems and methods for providing a sharedcharge in pixelated image detectors. One method includes providing aplurality of pixels for a pixelated solid-state photon detector in aconfiguration such that a charge distribution is detected by at leasttwo pixels and obtaining charge information from the at least twopixels. The method further includes determining a position of aninteraction of the charge distribution with the plurality of pixelsbased on the obtained charge information.

US 2015/0025852 relates to a method of making correct determination ofelectric charge collection among signals from a semiconductor radiationdetector.

US 2016/0124096 relates to x-ray detectors provided in a cross-stripgeometry with better resolution than the electrode spacing. The basicidea is analog charge cloud reconstruction by rotating the electrodepattern by about 45 degrees relative to the detector slab, whichprovides performance benefits such as equal length for all electrodesand greater ease of integration into vertical stacks.

US 2017/0212254 relates to a detector for a Compton camera including afirst radiation scattering layer; a second radiation scattering layer;and a radiation absorption layer disposed between the first radiationscattering layer and the second radiation scattering layer. The firstradiation scattering layer and the radiation absorption layer configureat least a part of a first detector, and the second radiation scatteringlayer and the radiation absorption layer configure at least a part of asecond detector.

SUMMARY

It is a general object to provide improvements related tophoton-counting x-ray detectors.

According to the proposed technology, it is desirable to provide amethod and corresponding system for enabling estimation of an initialpoint of interaction of an x-ray photon in a photon-counting x-raydetector, as well as a corresponding x-ray imaging system, computerprogram and computer-program product.

According to a first aspect, there is provided a method for enablingestimation of an initial point of interaction of an x-ray photon in aphoton-counting x-ray detector, which is based on a number of x-raydetector sub-modules or wafers, each of which comprises detectorelements, wherein the x-ray detector sub-modules are oriented in edge-ongeometry with the edge directed towards the x-ray source, assuming thex-rays enter through the edge. Each detector sub-module or wafer has athickness with two opposite sides of different potentials to enablecharge drift towards the side, where the detector elements, alsoreferred to as pixels, are arranged. Basically, the method comprises:

-   -   determining an estimate of charge diffusion originating from a        Compton interaction or an interaction through photoeffect        related to the x-ray photon in a detector sub-module or wafer of        the x-ray detector; and    -   estimating the initial point of interaction along the thickness        of the detector sub-module based on the determined estimate of        charge diffusion.

According to a second aspect, there is provided a system for enablingestimation of an initial point of interaction of an x-ray photon in aphoton-counting x-ray detector, which is based on a number of x-raydetector sub-modules or wafers, each of which comprises detectorelements, wherein the x-ray detector sub-modules are oriented in edge-ongeometry with the edge directed towards the x-ray source, assuming thex-rays enter through the edge. Each detector sub-module or wafer has athickness with two opposite sides of different potentials to enablecharge drift towards the side, where the detector elements, alsoreferred to as pixels, are arranged. Basically, the system is configuredto determine an estimate of charge diffusion originating from a Comptoninteraction or an interaction through photoeffect related to the x-rayphoton in a detector sub-module or wafer of the x-ray detector. Thesystem is also configured to estimate the initial point of interactionalong the thickness of the detector sub-module based on the determinedestimate of charge diffusion.

According to a third aspect, there is provided an x-ray imaging systemcomprising a system according to the second aspect.

According to a fourth aspect, there is provided a corresponding computerprogram and computer-program product.

In this way, x-ray imaging and/or image reconstruction can beconsiderably improved. For example, the resolution can be significantlyimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating an example of an overallx-ray imaging system.

FIG. 2 is a schematic diagram illustrating another example of an x-rayimaging system.

FIG. 3 is a schematic diagram illustrating an example of the conceptualstructure for implementing an energy-discriminating photon-countingdetector.

FIG. 4A is a schematic diagram illustrating an example of an x-raydetector sub-module according to an exemplary embodiment.

FIG. 4B is a schematic diagram illustrating an example of a modularx-ray detector comprising a number of detector sub-modules arrangedside-by-side, e.g. in a slightly curved overall geometry with respect toan x-ray source located at an x-ray focal point.

FIG. 4C is a schematic diagram illustrating an example of a modularx-ray detector comprising a number of detector sub-modules arrangedside-by-side, and also stacked one after the other.

FIG. 4D is a schematic diagram illustrating an example of aphoton-counting x-ray detector, which is based on a number of x-raydetector sub-modules, here referred to as wafers that are stacked oneafter the other.

FIG. 5 is a schematic diagram illustrating an example of the photoncount rate as a function of segment in a depth-segmented x-ray detector.

FIG. 6 is a schematic diagram illustrating the Compton effect.

FIG. 7A is a schematic diagram illustrating an example of pixels of aparticular wafer in the x-z plane.

FIG. 7B is a schematic diagram illustrating an example of a charge cloudprofile in the x-direction for a charge cloud.

FIG. 7C is a schematic diagram illustrating an example of a charge cloudprofile in the z-direction for a charge cloud.

FIG. 7D is a schematic diagram illustrating an example of how the widthof the charge diffusion or cloud is dependent on the distance, along thethickness of the considered detector sub-module or wafer of an x-raydetector, from the initial point of interaction to the point ofdetection.

FIG. 8 is a schematic flow diagram illustrating an example of a methodfor enabling estimation of an initial point of interaction of an x-rayphoton in a photon-counting x-ray detector.

FIG. 9 is a schematic diagram illustrating an example of an x-raydetector sub-module according to an embodiment.

FIG. 10 is a schematic diagram illustrating another example of an x-raydetector sub-module according to an embodiment.

FIG. 11 is a schematic diagram illustrating an example of an activeintegrated pixel according to an embodiment.

FIG. 12 is a schematic diagram illustrating another example of an activeintegrated pixel according to another embodiment.

FIG. 13 is a schematic diagram illustrating yet another example of anactive integrated pixel according to a further embodiment.

FIG. 14 is a schematic diagram illustrating still another example of anactive integrated pixel according to yet another embodiment.

FIG. 15 is a schematic diagram illustrating an example of a computerimplementation according to an embodiment.

DETAILED DESCRIPTION

In general, x-ray photons are converted to electron-hole pairs insidethe semiconductor material of the x-ray detector, where the number ofelectron-hole pairs is generally proportional to the photon energy. Theelectrons and holes are drifting towards the detector elements, thenleaving the photon-counting detector. During this drift, the electronsand holes induce an electrical current in the detector elements.

It is desirable to enable improved estimation of an initial point ofinteraction of an x-ray photon in a photon-counting x-ray detector,which is based on a number of x-ray detector sub-modules or wafers, eachof which comprises detector elements, wherein the x-ray detectorsub-modules are oriented in edge-on geometry with the edge directedtowards the x-ray source, assuming the x-rays enter through the edge.

Each detector sub-module or wafer has a thickness with two oppositesides, such as a front/main side and a back side, of differentpotentials to enable charge drift towards the (front/main) side, wherethe detector elements, also referred to as pixels, are arranged.

A basic idea is to determine an estimate of charge diffusion originatingfrom a Compton interaction or an interaction through photoeffect relatedto the x-ray photon in a (particular) detector sub-module or wafer ofthe x-ray detector, and estimate the initial point of interaction alongthe thickness of the detector sub-module at least partly based on thedetermined estimate of charge diffusion.

As mentioned, the x-ray detector sub-modules are oriented in edge-ongeometry with the edge directed towards the x-ray source, assuming thex-rays enter through the edge.

Edge-on is a design for an x-ray detector, where the x-ray sensors suchas x-ray detector elements or pixels are oriented edge-on to incomingx-rays.

As an example, each of the x-ray detector sub-modules may comprisedetector elements distributed over the detector sub-module or wafer intwo directions, including the direction of the incoming x-rays. Thisnormally corresponds to a so-called depth-segmented x-ray detectorsub-module. The proposed technology is however also applicable for usewith non-depth-segmented x-ray detector sub-modules. The detectorelements may be arranged as an array in a direction substantiallyorthogonal to the direction of the incident x-rays, while each of thedetector elements is oriented edge-on to the incident x-rays. In otherwords, the x-ray detector sub-module may be non-depth-segmented, whilestill arranged edge-on to the incoming x-rays.

In a particular example, at least part of the detector elements, orpixels, have a longer extension in a direction of the incident X-raysthan in a direction orthogonal to the direction of the incident X-rays,with a relation of at least 2:1. In other words, the detector elements,or pixels, may be asymmetric in the geometrical design and have at leastdouble the extension (depth) in the direction of the incident X-raysthan the extension in a direction orthogonal (perpendicular) to thedirection of the incident X-rays.

It should be understood that the estimate of charge diffusion may bedetermined, e.g. based on induced current caused by moving electron-holepairs originating from the Compton interaction or interaction throughphotoeffect, as detected by detector elements distributed over the x-raydetector sub-module or wafer.

In a particular example, the step of determining an estimate of chargediffusion comprises measuring or estimating the shape and/or width ofthe charge diffusion.

As an example, the charge diffusion may be represented by a chargecloud, and the estimate of charge diffusion may be determined bymeasuring or estimating the shape and/or width of the charge cloud. Asthe estimate of charge diffusion may be represented by the inducedcurrent, as detected by detector elements, the shape and/or width of thecharge cloud may hence be related to the measured or detected inducedcurrent.

Optionally, the initial point of interaction of the incident x-rayphoton along the thickness of the detector sub-module is estimated basedon the measured width of the cloud and the integrated charge of thecloud. As explained, a representation of the charge cloud may beprovided by the induced current on triggered detector elements of adetector sub-module.

By way of example, it may be possible to determine an estimate of adistance, along the thickness of the detector sub-module, between thepoint of detection of the x-ray photon in the detector sub-module andthe initial point of interaction based on the estimate of chargediffusion, and then determine an estimate of the initial point ofinteraction based on the point of detection and the determined estimateof a distance along the thickness of the detector sub-module.

The interaction is an interaction between the x-ray photon and thesemiconductor substrate (typically made of silicon).

The thickness of the detector sub-module or wafer generally extendsbetween the two opposite sides, such as the back side and front side, ofthe detector sub-module.

By way of example, the shape, and in particular, the width of the chargediffusion is measured or estimated, and the distance between the pointof detection and the initial point of interaction is determined based onthe shape or width of the charge diffusion or distribution.

For example, the detector elements distributed over the detectorsub-module or wafer on the front side provide an array of pixels, wherethe pixels are generally smaller than the charge cloud to be resolved.

In a particular example, the two directions over which the detectorelements are distributed on the front/main side of the considereddetector sub-module or wafer typically include the length and depthdirections of the detector sub-module. The direction of the incomingx-rays generally corresponds to the depth direction and this is thereason for calling this type of x-ray detector a depth-segmented x-raydetector or edge-on x-ray detector.

By way of example, it is possible to have a design in which an estimateof charge diffusion can be determined in each of a number of particulardetector sub-modules or wafers, and wherein an estimate of the point ofinteraction of the incident x-ray photon in the corresponding orrespective detector sub-module can be determined. This can also beperformed for each of a number of incident x-ray photons.

In other words, the method is performed for determining, for each of anumber of incident x-ray photons and/or each of a number of x-raydetector sub-modules, a corresponding estimate of charge diffusion, andfor determining an estimate of the initial point of interaction of theincident x-ray photon in the respective x-ray detector sub-module.

The proposed technology offers considerable improvements for x-rayimaging and/or image reconstruction, more specifically significantlyincreased resolution.

Information about the charge diffusion may also be used for providingimproved resolution in at least one of the two directions over which thedetector elements are distributed on the front side of the detectorsub-module or wafer. For example, increased resolution may be obtainedby more accurately determining a point of interaction based oninformation of a charge cloud profile in one or both of thesedirections. The considered direction(s) may include the length and/ordepth directions of the detector sub-module or wafer.

By way of example, the depth-segmented x-ray detector may include aplurality of detector sub-modules, each of which has a number of depthsegments of detector elements in the direction of the incoming x-rays.

For example, the detector sub-modules may be arranged one after theother and/or arranged side-by-side in a configuration to form aneffective detector area or volume.

FIG. 4A is a schematic diagram illustrating an example of an x-raydetector sub-module according to an exemplary embodiment. In thisexample, the sensor part of the x-ray detector sub-module 21 is dividedinto so-called depth segments in the depth direction, assuming thex-rays enter through the edge. Each detector element 22 is normallybased on a diode having a charge collecting electrode as a keycomponent.

Normally, a detector element is an individual x-ray sensitivesub-element of the detector. In general, the photon interaction takesplace in a detector element and the thus generated charge is collectedby the corresponding electrode of the detector element. Each detectorelement typically measures the incident x-ray flux as a sequence offrames. A frame is the measured data during a specified time interval,called frame time.

FIG. 4B is a schematic diagram illustrating an example of a modularx-ray detector comprising a number of detector sub-modules 21 arrangedside-by-side, e.g. in a slightly curved overall geometry with respect toan x-ray source located at an x-ray focal point.

FIG. 4C is a schematic diagram illustrating an example of a modularx-ray detector comprising a number of detector sub-modules 21 arrangedside-by-side, and also stacked one after the other. The x-ray detectorsub-modules may be stacked one after the other to form larger detectormodules that may be assembled together side-by-side to build up anoverall x-ray detector system.

As mentioned, edge-on is a design for an x-ray detector, where the x-raysensors such as x-ray detector elements or pixels are oriented edge-onto incoming x-rays.

For example, the detector may have detector elements in at least twodirections, wherein one of the directions of the edge-on detector has acomponent in the direction of the x-rays. Such an edge-on detector issometimes referred to as a depth-segmented x-ray detector, having two ormore depth segments of detector elements in the direction of theincoming x-rays.

Alternatively, the x-ray detector may be non-depth-segmented, whilestill arranged edge-on to the incoming x-rays.

Depending on the detector topology, a detector element may correspond toa pixel, e.g. when the detector is a flat-panel detector. However, adepth-segmented detector may be regarded as having a number of detectorstrips, each strip having a number of depth segments. For such adepth-segmented detector, each depth segment may be regarded as anindividual detector element, especially if each of the depth segments isassociated with its own individual charge collecting electrode.

The detector strips of a depth-segmented detector normally correspond tothe pixels of an ordinary flat-panel detector. However, it is alsopossible to regard a depth-segmented detector as a three-dimensionalpixel array, where each pixel (sometimes referred to as a voxel)corresponds to an individual depth segment/detector element. Photoncounting detectors have emerged as a feasible alternative in someapplications; currently those detectors are commercially availablemainly in mammography. The photon counting detectors have an advantagesince in principle the energy for each x-ray can be measured whichyields additional information about the composition of the object. Thisinformation can be used to increase the image quality and/or to decreasethe radiation dose.

Compared to the energy-integrating systems, photon-counting CT has thefollowing advantages. Firstly, electronic noise that is integrated intothe signal by the energy-integrating detectors can be rejected bysetting the lowest energy threshold above the noise floor in thephoton-counting detectors. Secondly, energy information can be extractedby the detector, which allows improving contrast-to-noise ratio byoptimal energy weighting and which also allows so-called material basisdecomposition, by which different materials and/or components in theexamined subject or object can be identified and quantified, to beimplemented effectively. Thirdly, more than two basis materials can beused which benefits decomposition techniques, such as K-edge imagingwhereby distribution of contrast agents, e.g. iodine or gadolinium, arequantitatively determined. Fourth, there is no detector afterglow,meaning that high angular resolution can be obtained. Last but notleast, higher spatial resolution can be achieved by using smaller pixelsize.

A problem in any counting x-ray photon detector is the so-called pile-upproblem. When the flux rate of x-ray photons is high there may beproblems in distinguishing between two subsequent charge pulses. Asmentioned above, the pulse length after the filter depends on theshaping time. If this pulse length is larger than the time between twox-ray photon induced charge pulses, the pulses will grow together, andthe two photons are not distinguishable and may be counted as one pulse.This is called pile-up. One way to avoid pile-up at high photon flux isthus to use a small shaping time, or to use depth-segmentation assuggested in optional embodiments described herein.

In order to increase the absorption efficiency, the detector canaccordingly be arranged edge-on, in which case the absorption depth canbe chosen to any length and the detector can still be fully depletedwithout going to very high voltages.

In particular, silicon has many advantages as detector material such ashigh purity and a low energy required for creation of charge carriers(electron-hole pairs) and also a high mobility for these charge carrierswhich means it will work even for high rates of x-rays.

The semiconductor x-ray detector sub-modules are normally tiled togetherto form a full detector of almost arbitrary size with almost perfectgeometrical efficiency except for an optional anti-scatter module whichmay be integrated between at least some of the semiconductor detectormodules.

More information on so-called photon-counting edge-on x-ray detectors ingeneral can be found, e.g. in U.S. Pat. No. 8,183,535, which disclosesan example of a photon-counting edge-on x-ray detector. In U.S. Pat. No.8,183,535, there are multiple semiconductor detector modules arrangedtogether to form an overall detector area, where each semiconductordetector module comprises an x-ray sensor that is oriented edge-on toincoming x-rays and connected to integrated circuitry for registrationof x-rays interacting in the x-ray sensor.

As discussed, an overall x-ray detector may for example be based ondetector sub-modules, or wafers, each of which has a number of depthsegments in the direction of the incoming x-rays.

Such detector sub-modules can then be arranged or stacked one after theother and/or arranged side-by-side in a variety of configurations toform any effective detector area or volume. For example, a full detectorfor CT applications typically has a total area greater than 200 cm²,which results in a large number of detector modules, such as 1500-2000detector modules.

By way of example, detector sub-modules may generally be arrangedside-by-side and/or stacked, e.g. in a planar or slightly curved overallconfiguration.

In general, it is desirable that incoming x-rays have the chance to passthrough as many detector elements or segments as possible to provide asmuch spatial/energy information as possible.

Since the x-ray interactions will be distributed and occurring indifferent depth segments along the depth (length) of the sensor, theoverall count rate will be distributed among the segments in depth, e.g.as can be seen from FIG. 5, which is a schematic diagram illustrating anexample of the count rate in each segment. In this example, the firstsegment is the segment closest to the x-ray source.

By way of example, over a 40 mm deep sensor it would be possible to have400 segments or more and the count rate would be correspondinglydecreased. The sensor depth is vital for dose efficiency and thesegmentation protects from pulse pile-up and maintains the spatialresolution of the system.

By way of example, the current may be measured, e.g., through a ChargeSensitive Amplifier (CSA), followed by a Shaping Filter (SF), e.g. asschematically illustrated in previously mentioned FIG. 3.

As the number of electrons and holes from one x-ray event isproportional to the x-ray energy, the total charge in one inducedcurrent pulse is proportional to this energy. The current pulse isamplified in the CSA and then filtered by the SF filter. By choosing anappropriate shaping time of the SF filter, the pulse amplitude afterfiltering is proportional to the total charge in the current pulse, andtherefore proportional to the x-ray energy. Following the SF filter, thepulse amplitude may be measured by comparing its value with one orseveral threshold values (T₁-T_(N)) in one or more comparators (COMP),and counters are introduced by which the number of cases when a pulse islarger than the threshold value may be recorded. In this way it ispossible to count and/or record the number of x-ray photons with anenergy exceeding an energy corresponding to respective threshold value(T₁-T_(N)) which has been detected within a certain time frame.

When using several different threshold values, a so-calledenergy-discriminating photon-counting detector is obtained, in which thedetected photons can be sorted into energy bins corresponding to thevarious threshold values. Sometimes, this particular type ofphoton-counting detector is also referred to as a multi-bin detector.

In general, the energy information allows for new kinds of images to becreated, where new information is available and image artifacts inherentto conventional technology can be removed.

In other words, for an energy-discriminating photon-counting detector,the pulse heights are compared to a number of programmable thresholds(T₁-T_(N)) in the comparators and classified according to pulse-height,which in turn is proportional to energy.

However, an inherent problem in any charge sensitive amplifier is thatit will add electronic noise to the detected current. In order to avoiddetecting noise instead of real X-ray photons, it is therefore importantto set the lowest threshold value high enough so that the number oftimes the noise value exceeds the threshold value is low enough not todisturb the detection of X-ray photons.

By setting the lowest threshold above the noise floor, electronic noise,which is the major obstacle in the reduction of radiation dose of theX-ray imaging systems, can be significantly reduced

The shaping filter has the general property that large values of theshaping time will lead to a long pulse caused by the x-ray photon andreduce the noise amplitude after the filter. Small values of the shapingtime will lead to a short pulse and a larger noise amplitude. Therefore,in order to count as many X-ray photons as possible, a large shapingtime is desired to minimize noise and allowing the use of a relativelysmall threshold level.

The values of the set or table of thresholds, by which the pulse heightsare compared in the comparators, affect the quality of the image datagenerated by the photon-counting detector. Furthermore, these thresholdvalues are temperature dependent. Therefore, in an embodiment, thecalibration data generated by the power-consuming circuitries is a setor table or thresholds (T₁-T_(N)).

It should though be understood that it is not necessary to have anenergy-discriminating photon-counting detector, although this comes withcertain advantages.

FIG. 4D is a schematic diagram illustrating an example of aphoton-counting x-ray detector, which is based on a number of x-raydetector sub-modules 21, here referred to as wafers. The wafers 21 arestacked one after the other. It can be seen that each wafer has a length(x) and a thickness (y), and that each wafer is also segmented in thedepth direction (z), so-called depth segmentation. Purely as an example,the length of the wafer may be in order of 25-50 mm, and the depth ofthe wafer may be in the same order of 25-50 mm, whereas the thickness ofthe wafer may be in the order of 300-900 um.

By way of example, each wafer has detector elements distributed over thewafer in two directions including the direction of the incoming x-rays(z).

Each wafer has a thickness (y) with two opposite sides, such as a frontside and a back side, of different potentials to enable charge drifttowards the side, where the detector elements, also referred to aspixels, are normally arranged.

For a better understanding of the proposed technology it may be usefulto recall the basic concept of the Compton effect.

The incoming X-ray photons may interact with the semiconductor materialof the detector modules either through the photoelectric effect, simplyreferred to as the photoeffect herein, or Compton interaction, see FIG.6 Compton interaction, also referred to as Compton scattering, is thescattering of a photon by a charged particle, usually an electron. Itresults in a decrease in energy of the photon, called the Comptoneffect. Part of the energy of the photon is transferred to the recoilingelectron. The photon may be involved in multiple Compton interactionsduring its path through the semiconductor substrate. Briefly, in aCompton interaction, an incident x-ray photon is deflected from itsoriginal path by an interaction with an electron, which is ejected fromits initial orbital position to form a so-called secondary or “free”electron. Such a secondary electron can also be the result of thephotoeffect, in which case the entire energy of the incident x-rayphoton is transferred to the electron.

More specifically, an x-ray photon may create a secondary electronthrough Compton interaction or photoeffect. The electron will getkinetic energy from the x-ray photon and move a short distance, e.g. 1um-50 um, and during its path will excite electron-hole pairs. Everyelectron hole pair will cost about 3.6 eV to create which means that forexample a Compton interaction with 15 keV deposited energy to theelectron will create around 4200 electron-hole pairs, forming aso-called charge cloud. The cloud will move or drift according to theelectric field lines and if the backside of the detector sub-module orwafer is biased positive the holes will move towards the readoutelectrodes arranged on the front side of the detector sub-module orwafer and the electrons will move towards the back side. During drift,the electron-hole pairs forming the charge cloud will also be subject todiffusion, which basically means that the charge cloud will increase insize.

The readout electrodes are functioning as detector elements or pixels.By way of example, the voltage on the back side may be around 200 V andvirtual ground on the front side.

As should be understood, it is proposed to orient the x-ray detectoredge-on relative to the beam (i.e. edge-on relative to the incomingx-rays), while sub-dividing the sensor area into a relatively highresolution, e.g. into 5 um to 100 um resolution, in order to be able toresolve a charge cloud.

FIG. 7A is a schematic diagram illustrating an example of some of thepixels of a particular wafer in the x-z plane. In this example, thepixels 22 are generally smaller than the charge cloud to be resolved.For example, the charge cloud may have a width in the order of 100 um,and the pixels are therefore normally designed to be smaller or evenconsiderably smaller than that. Hence, an x-ray photon traveling throughthe semiconductor substrate typically results in a charge cloud coveringmultiple neighboring pixels in the detector module. This means that asingle x-ray photon will most likely trigger event detection in multiplepixels.

Although the pixels 22 are illustrated as squares, it should beunderstood that the pixels may be rectangular or have other forms.

According to a complementary aspect, information about the chargediffusion may be used for providing improved resolution in at least oneof the two directions over which the detector elements are distributedon the front side of the detector sub-module or wafer. For example,increased resolution may be obtained based on information of a chargecloud profile in one or both of these directions. The considereddirection(s) may include the length (x) direction and/or depth (z)direction of the detector sub-module or wafer.

By way of example, the method therefore further comprises the step ofdetermining an estimate of the point of interaction of the incidentx-ray photon in at least one of the two directions (x, z) over which thedetector elements are distributed on a main side of the x-ray detectorsub-module or wafer.

For example, the step of determining an estimate of the point ofinteraction of the incident x-ray photon in at least one of the twodirections (x, z) over which the detector elements are distributed onthe main side may be performed based on information of a charge cloudprofile in one or both of the two directions (x, z) over which thedetector elements are distributed on the main side of the x-ray detectorsub-module or wafer.

FIG. 7B is a schematic diagram illustrating an example of a charge cloudprofile in the x-direction for a charge cloud.

FIG. 7C is a schematic diagram illustrating an example of a charge cloudprofile in the z-direction for a charge cloud.

As an example, this may involve determining one or more charge cloudprofiles (e.g. see FIG. 7B and FIG. 7C) and performing curve fittingthrough any standard curve fitting methods such as weighted averagingand/or least mean square methods. For example, finding out where thecurve has its peak and identifying the peak as the point of interactionin a particular direction, can improve the resolution considerably, evendown to sub-pixel resolution, e.g. down to 1 um resolution. This can becompared to the spatial resolution of conventional x-ray imagingsystems, which may have a resolution of approximately 1 mm.

Alternatively, it may be possible to use information on which pixel 22that has detected the highest charge as the point of interaction. Forexample, the step of determining an estimate of the point of interactionof the incident x-ray photon in at least one of the two directions (x,z) over which the detector elements are distributed on the main side maybe performed by identifying the pixel that has detected the highestcharge as the point of interaction.

It should though be understood that with a proper curve fitting, asdescribed above, it may be possible to obtain sub-pixel resolution.

As previously indicated, the inventors have realized that the point ofdetection of a photon may differ quite significantly from the initialpoint of interaction, along the thickness (y) of the detector sub-moduleor wafer.

After careful analysis and experiments, the inventors have furtherrecognized that the shape, and in particular, the width of the chargediffusion or cloud is dependent on the distance, along the thickness ofthe considered detector sub-module or wafer of an x-ray detector, fromthe initial point of interaction to the point of detection. This isschematically shown in FIG. 7D for three different distances or depths(100 μm, 300 μm and 600 μm).

By way of example, if the charge cloud is not circular in cross-sectionbut rather elliptical or of other forms, and thereby has differentextensions in the different directions in the z-x plane, it isrecommendable to use the smallest width of the charge cloudcross-section as a relevant measure of the charge diffusion.

During the movement of the charge cloud the charges will diffuse andthis is accelerated by electrostatic repulsion. The induced current isdominated by movement of charge that occurs close to the front side.Since the diffusion is a function of time, the charge cloud will bewider (upon collection at the front side) if the interaction took placeclose to the back side (longer time) compared to close to the front side(negligible diffusion for contributing charge carriers). Knowing thetotal energy (integrated charge of the electron hole cloud) and thewidth of the cloud will enable an estimation of the point of interactionalong the thickness of the edge-on wafer.

The area of the photon-counting detector, in which coincidental or nearsimultaneous events are detected in neighboring detector elements (inthe x-y plane), thereby also gives depth information (in thez-direction) indicating the point of interaction between an incidentx-ray photon and the semiconductor material. Thus, the larger the areaof detection the wider the charge diffusion, implying a more remotepoint of interaction (such as 600 μm) as compared to the case with asmaller area of detection and narrow charge diffusion (such as 100 μm),as schematically illustrated in FIG. 7D. Experiments have shown that theresolution may be considerably improved, e.g. down to 50 um. This is aconsiderable improvement compared to simply knowing in which wafer theinteraction took place. It is now also possible to know, within aresolution of approximately 50 um, where along the thickness of thewafer the initial point of interaction occurred.

In some sense, the proposed technology can thus be considered asdirected towards estimating the point of interaction of an incidentx-ray photon in the semiconductor material by determining or measuringor otherwise estimating the charge diffusion and distributionoriginating from a Compton interaction or an interaction throughphotoeffect during the path through the semiconductor material. Theinformation with regard to the point of interaction may then be used bythe x-ray imaging system in x-ray imaging and/or image reconstruction.

FIG. 8 is a schematic flow diagram illustrating an example of a methodfor enabling estimation of an initial point of interaction of an x-rayphoton in a photon-counting x-ray detector. The photon-counting x-raydetector is based on a number of x-ray detector sub-modules or wafers,each of which comprises detector elements distributed over the detectorsub-module or wafer in two directions including the direction of theincoming x-rays.

Each detector sub-module or wafer has a thickness with two oppositesides, such as a front side and a back side, of different potentials toenable charge drift towards the side, where the detector elements, alsoreferred to as pixels, are arranged.

Basically, the method comprises:

-   S1: determining an estimate of charge diffusion originating from a    Compton interaction or an interaction through photoeffect related to    the x-ray photon in a detector sub-module or wafer of the x-ray    detector; and-   S2: estimating the initial point of interaction along the thickness    of the detector sub-module based on the determined estimate of    charge diffusion.

In this way, x-ray imaging and/or image reconstruction can beconsiderably improved. For example, the resolution can be significantlyimproved.

By way of example, it may be possible to determine an estimate of adistance, along the thickness of the detector sub-module, between thepoint of detection of the x-ray photon in the detector sub-module andthe initial point of interaction based on the estimate of chargediffusion, and then determine an estimate of the initial point ofinteraction based on the point of detection and the determined estimateof a distance along the thickness of the detector sub-module.

In other words, the step of estimating the initial point of interactionalong the thickness of the detector sub-module may comprise thefollowing steps:

-   -   determining an estimate of a distance, along the thickness of        the x-ray detector sub-module, between the point of detection of        the x-ray photon in the x-ray detector sub-module and the        initial point of interaction based on the estimate of charge        diffusion, and    -   determining the estimate of the initial point of interaction        based on the point of detection and the determined estimate of a        distance along the thickness of the detector sub-module.

The thickness of the detector sub-module or wafer generally extends inthe drift direction between the back side and front side of the detectorsub-module.

By way of example, the shape, and in particular, the width of the chargediffusion is measured or estimated, and the distance between the pointof detection and the initial point of interaction is determined based onthe shape or width of the charge diffusion or distribution.

For example, the charge diffusion may be represented by a charge cloud,and the detector elements distributed over the detector sub-module orwafer on a main side may provide an array of pixels, where the pixelsare generally smaller than the charge cloud to be resolved.

In the following, non-limiting examples of embodiments for providingx-ray detector sub-modules and pixels suitable for the proposedtechnology will be described.

In the prior art, it has been suggested to implement the detectorsub-modules, sometimes simply referred to as detector modules, asso-called Multi-Chip Modules (MCMs) in the sense that the detectormodules have semiconductor base substrates for electric routing and fora number of ASICs. The routing will include a connection for the signalfrom each pixel to an ASIC input as well as connections from the ASICsto external memory and/or digital data processing. Power to the ASICsmay be provided through similar routing taking into account the increasein cross-section which is required for the large currents in theseconnections, but the power may also be provided through a separateconnection. Hence, in the prior art, each individual pixel is connectedto a subsequent ASIC channel where an MCM technology is employed tointegrate the ASICs and electric routing on the silicon substrate.

The proposed technology provides further improvements over prior artx-ray detectors by using active integrated pixels in the detectormodules. This means that part of the analog processing of the electricsignals is moved from the ASICs into the pixels. For instance, movingthe pre-amplifying from the ASICs to the pixels lower the capacitance atthe input to the pre-amplifiers since no long traces are needed to routethe signal from the pixels to the ASICs. Further advantages ofintegrating at least part of the analog signal processing in the pixelsinclude smaller pixel sizes, which in turn reduces the power consumptionper pixel and enables a reduction of the minimum noise threshold.

FIG. 9 is a schematic diagram of a detector module, also referred to asa chip or wafer, according to an embodiment. In this example, thedetector module 21 comprises a semiconductor substrate or materialcomprising a plurality of active integrated pixels arranged in thesemiconductor substrate. In a particular embodiment, the plurality ofactive integrated pixels is arranged at a main side (front side) of thesemiconductor substrate in a grid or matrix, or other pattern, as shownin the figure. The figure also illustrates the arrangement of the pixelsin different depth segments with regard to the edge facing the X-raysource and at which X-rays incident on the detector module.

In an embodiment, the detector module also comprises further processingcircuitry, such as analog processing circuitry and/or digital processingcircuitry, exemplified as read-out circuitry, control circuitry andanalog-to-digital conversion (ADC) circuitry in the figure. Thesefurther processing circuitry may be implemented in or as one or moreASICs.

The further processing circuitry is advantageously arranged in thesemiconductor substrate at the same main side (front side) as theplurality of active integrated pixels. In such a case, the furtherprocessing circuitry is preferably arranged at the portion or part ofthe main side at or in connection with the edge facing away from theX-ray source and the incident x-ray as shown in the figure. Thisembodiment reduces any dead area of the detector module by reducing theportion of the detector module that is used for the further processingcircuitry. In addition, the further processing circuitry is protectedfrom the incoming X-ray by be arranged furthest away from the edge ofincidence.

In an illustrative, but non-limiting, example the area of thesemiconductor substrate comprising active integrated pixels may be from5×5 mm up to 50×50 mm, such as 10×10 mm, 15×15 mm, 20×20 mm, 25×25 mm,30×30 mm, 35×35 mm, 40×40 mm or 45×45 mm. Also, non-quadratic, such asrectangular, areas with active integrated pixels are possible.

In FIG. 4D, each wafer may comprise one detector module or may comprisemultiple detector modules. In the latter case, the detector modules maybe attached to a thin substrate, such as a ceramic substrate, to form awafer that can be handled as a single unit. Sometime, this single unitmay itself be referred to as a detector module, or a detectorsub-module. Purely as an example, the width of the wafer may be in orderof 25-50 mm, and the depth of the wafer may be in the same order of25-50 mm, whereas the thickness of the wafer may be in the order of300-900 μm.

FIG. 9 schematically also indicates an active integrated pixel with aso-called detector diode (electrode) together with read-out electronicsand interconnections. Each such active integrated pixel typically has asize in the μm range. In an embodiment, the active integrated pixels arequadratic and typically all active integrated pixels in a detectormodule have the same shape and size. It is, however, possible to useother shapes for the pixels, such as rectangular, and/or having activeintegrated pixels with different sizes and/or shapes in the samedetector module as shown in FIG. 10. In FIG. 10, the active integratedpixels have the same width but different depths. For instance, the depthof the active integrated pixels may increase for different depth segmentand thereby based on the distance to the edge at which the X-raysincident on the detector module. This means that the active integratedpixels at this edge preferably have smaller depth as compared to activeintegrated pixels closest to the opposite edge. In such an embodiment,the detector modules may include active integrated pixels having two ormore different depths.

Different pixel depths, and in particular pixel depth as a function ofdepth segment or distance to the edge at which the X-rays incident onthe detector module can be used to tailor the probabilities orlikelihoods for detecting an event at an active integrated pixel.

According to a specific aspect of the proposed technology, all or partof the analog signal processing illustrated in FIG. 3 may be integratedinto the pixels to thereby form so-called active integrated pixels.

As mentioned, an aspect of the invention relates to an edge-onphoton-counting detector. The edge-on photon-counting detector comprisesat least one detector module having a respective edge facing incidentX-rays. The at least one detector module comprises a semiconductorsubstrate.

In a particular example, the edge-on photon-counting detector alsocomprises a plurality of active integrated pixels arranged in thesemiconductor substrate.

In an embodiment, the edge-on photon-counting detector comprisesmultiple detector modules arranged side-by-side and/or stacked.

The edge-on photon-counting detector is typically fabricated based onsilicon as semiconductor material for the detector modules.

To compensate for the low stopping power of silicon, the detectormodules are typically oriented in edge-on geometry with their edgedirected towards the X-ray source, thereby increasing the absorptionthickness. In order to cope with the high photon fluxes in clinical CT,a segmented structure of the active integrated pixels into depthsegments is preferably applied, which is achieved by implantingindividual active integrated pixels in depth segments on the siliconsubstrate.

In a particular embodiment, the semiconductor substrate is made of floatzone (FZ) silicon. FZ silicon is very pure silicon obtained by verticalzone melting. In the vertical configuration molten silicon hassufficient surface tension to keep the charge from separating. Avoidanceof the necessity of a containment vessel prevents contamination of thesilicon. Hence, the concentrations of light impurities in the FZ siliconare extremely low. The diameters of FZ silicon wafers are generally notgreater than 200 mm due to the surface tension limitations duringgrowth. A polycrystalline rod of ultra-pure electronic grade silicon ispassed through an RF heating coil, which creates a localized molten zonefrom which the crystal ingot grows. A seed crystal is used at one end inorder to start the growth. The whole process is carried out in anevacuated chamber or in an inert gas purge. The molten zone carries theimpurities away with it and, hence, reduces impurity concentration.Specialized doping techniques like core doping, pill doping, gas dopingand neutron transmutation doping may be used to incorporate a uniformconcentration of impurity.

The semiconductor substrate is, in an embodiment, made of highresistivity silicon, such as high resistivity FZ silicon. As usedherein, high resistivity silicon is defined as monocrystalline siliconhaving a bulk resistivity larger than 1 kΩcm.

The plurality of active integrated pixels may be implemented as activeintegrated Complementary Metal Oxide Semiconductor (CMOS) pixels in thesemiconductor substrate. Hence, the analog circuitry of the activeintegrated pixels may be produced using CMOS technology.

FIGS. 11 to 14 illustrate various embodiments of such active integratedpixels with different analog read-out electronics in the pixels. Inthese figures, the current generating part of the pixel is illustratedas a diode outputting a current pulse or diode signal.

FIG. 11 illustrates an embodiment of an active integrated pixelcomprising an amplifier configured to generate an output signal based ona current pulse generated by the active integrated pixel or diode. In anembodiment, the amplifier is a charge sensitive amplifier (CSA)configured to integrate the current pulse into a voltage signal.

The output signal, such as voltage signal, from the amplifier,preferably CSA, is in this embodiment routed to external processingcircuitry arranged in the semiconductor substrate in the detectormodule, such as in the form of one or more ASICS, see read-out, ctrl andADC in FIGS. 9 and 10.

With an increased number of active integrated pixels in the detectormodule the count rate per pixel decreases and also the noiserequirements are relaxed. This implies that amplifiers withcomparatively low power consumption and low bandwidth can be used in theactive integrated pixels. Furthermore, single-ended amplifiers arepreferred due to the nature of the diode. This further allows for lesscomplex amplifiers. The lower diode capacitance, the input referrednoise from the amplifier will be less dominant as compared to usinglarger pixel sizes.

FIG. 12 illustrates another embodiment of an active integrated pixel.This embodiment comprises a pulse shaper, also referred to as shapingfilter, in addition to the amplifier. This pulse shaper is configured tofilter the output signal from the amplifier.

The current pulse from the diode is preferably integrated using a CSA.Typically, this generates a slow-moving voltage at the output of theCSA. To compensate for this behavior a cancellation circuit (CC), suchas a pole-zero cancellation circuit, is arranged connected to the CSAand the pulse shaper. This pole-zero CC cancels or at least suppressesthe slow response of the CSA with maintained charge/current integration.Accordingly, the time constant will instead be determined by the shaperintegration time of the pulse shaper.

The output signal from the pulse shaper is in this embodiment routed toexternal processing circuitry arranged in the semiconductor substrate inthe detector module, such as in the form of one or more ASICS, seeread-out, ctrl and ADC in FIGS. 9 and 10.

FIG. 13 illustrates a further embodiment of an active integrated pixel.This embodiment comprise an analog storage connected to, and arrangeddownstream of, the pulse shaper. This analog storage could beimplemented in the active integrated pixel to at least temporarily storeand retain the output signal from the pulse shaper. This enablescontrolled read-out of data from the active integrated pixel and theanalog storage, such as based on a control signal (ctrl) and or atscheduled time instances, such as controlled based on a clock signal(clk).

An analog storage as shown in FIG. 13 may also be used in an embodimentas shown in FIG. 11, i.e., without any pulse shaper. In such a case, theanalog storage is connected to the amplifier (CSA) or connected to theamplifier (CSA) through the pole-zero CC.

In yet another embodiment as shown in FIG. 14, the pixel comprises anevent detector represented as a comparator in the figure. This eventdetector is then configured to detect a photon event by comparing apulse amplitude of the output signal from the pulse shaper with athreshold value, represented by a noise threshold in the figure.

In a particular embodiment, the event detector is configured to generatea trigger signal based on the comparison of the pulse amplitude with thethreshold value, and preferably generates the trigger signal if thepulse amplitude is equal to or exceeds, or exceeds, the threshold value.

In this embodiment, read-out of the analog storage may be controlled bythe trigger signal output by the event detector. Thus, read-out of thedata in the analog storage then takes place preferably only when theevent detector confirms detection of a photon event by the activeintegrated pixel as represented by having a pulse amplitude (equal toor) above a noise floor as represented by the noise threshold.

In other words, a comparator acting as an event detector can be used tosignal to read-out circuitry, typically arranged externally relative tothe active integrated pixel, see read-out in FIGS. 9 and 10. Thisread-out circuitry reads the analog storage based on the trigger signalfrom the event detector. The read data may then be further processed,such as compared to thresholds (T₁-T_(N)), see FIG. 3, and/or digitizedin an ADC, see FIGS. 9 and 10.

If no read-out of the data in the analog storage is performed the datatherein may be consecutively flushed, such as by operating in afirst-in-first-out (FIFO) manner. This allows for an asynchronous readout of the data from the analog storage and thereby a reduction in thepower consumption during read out.

The trigger signal from the event detector may also be fed toneighboring active integrated pixels in the detector module to triggerthem to store data that may then be read out and further processed. Thisenables detection of properties of the data even through the noisethresholding is not passed.

In another embodiment, read out of the analog storage is performed basedon not only a trigger signal from the event detector in the activeintegrated pixel but also from a respective trigger signal from at leastone neighboring active integrated pixel in the detector module.

Implementations of active integrated pixels enable a reduction in sizeof the pixels as compared to prior art solutions. This small size of theactive integrated pixels allows multiple active integrated pixels in adetector sub-module to detect a charge cloud generated by a single x-rayphoton. This in turn enables determination of an estimate of chargediffusion originating from a Compton interaction or an interactionthrough photoeffect related to the X-ray photon in a particular detectorsub-module of the edge-on photon-counting detector, and estimation ofthe initial point of interaction of the x-ray photon along the thicknessof the detector sub-module at least partly based on the determinedestimate of charge diffusion, e.g. as previously described.

It will be appreciated that the methods and devices described herein canbe combined and re-arranged in a variety of ways.

For example, specific functions may be implemented in hardware, or insoftware for execution by suitable processing circuitry, or acombination thereof.

The steps, functions, procedures, modules and/or blocks described hereinmay be implemented in hardware using any conventional technology, suchas semiconductor technology, discrete circuit or integrated circuittechnology, including both general-purpose electronic circuitry andapplication-specific circuitry.

Particular examples include one or more suitably configured digitalsignal processors and other known electronic circuits, e.g. discretelogic gates interconnected to perform a specialized function, orApplication Specific Integrated Circuits (ASICs).

Alternatively, at least some of the steps, functions, procedures,modules and/or blocks described herein may be implemented in softwaresuch as a computer program for execution by suitable processingcircuitry such as one or more processors or processing units.

Examples of processing circuitry includes, but is not limited to, one ormore microprocessors, one or more Digital Signal Processors (DSPs), oneor more Central Processing Units (CPUs), video acceleration hardware,and/or any suitable programmable logic circuitry such as one or moreField Programmable Gate Arrays (FPGAs), or one or more ProgrammableLogic Controllers (PLCs).

It should also be understood that it may be possible to re-use thegeneral processing capabilities of any conventional device or unit inwhich the proposed technology is implemented. It may also be possible tore-use existing software, e.g. by reprogramming of the existing softwareor by adding new software components.

According to a second aspect, there is provided a system for enablingestimation of an initial point of interaction of an x-ray photon in aphoton-counting x-ray detector, which is based on a number of x-raydetector sub-modules or wafers, each of which comprises detectorelements, wherein the x-ray detector sub-modules are oriented in edge-ongeometry with the edge directed towards the x-ray source, assuming thex-rays enter through the edge.

Each detector sub-module or wafer has a thickness with two oppositesides of different potentials to enable charge drift towards the side,where the detector elements, also referred to as pixels, are arranged.

Basically, the system is configured to determine an estimate of chargediffusion originating from a Compton interaction or an interactionthrough photoeffect related to the x-ray photon in a detector sub-moduleor wafer of the x-ray detector. The system is also configured estimatethe initial point of interaction along the thickness of the detectorsub-module based on the determined estimate of charge diffusion.

As mentioned, the x-ray detector sub-modules may be oriented in edge-ongeometry with the edge directed towards the x-ray source, assuming thex-rays enter through the edge.

As an example, each of the x-ray detector sub-modules may comprisedetector elements distributed over the detector sub-module or wafer intwo directions, including the direction of the incoming x-rays. Thisnormally corresponds to a so-called depth-segmented x-ray detectorsub-module. The proposed technology is however also applicable for usewith non-depth-segmented x-ray detector sub-modules, as previouslyexplained.

By way of example, the system may be configured to determine, for eachof a number of incident x-ray photons and/or each of a number of x-raydetector sub-modules, a corresponding estimate of charge diffusion, andto determine an estimate of the initial point of interaction of theincident x-ray photon in the respective x-ray detector sub-module.

In a particular example, the system is configured to determine theestimate of charge diffusion by measuring or estimating the shape and/orwidth of the charge diffusion.

As an example, the charge diffusion may be represented by a chargecloud, and the system may be configured to determine the estimate ofcharge diffusion by measuring or estimating the shape and/or width ofthe charge cloud.

As an alternative or complementary example, the charge diffusion may berepresented by the induced current resulting from the drift of releasedelectron-hole pairs that originate from a Compton interaction orinteraction through photoeffect, as detected by detector elementsdistributed over the x-ray detector sub-module or wafer.

For example, the system may be configured to estimate the initial pointof interaction of the incident x-ray photon along the thickness of thedetector sub-module based on the measured width of the cloud and theintegrated charge of the cloud.

In a particular example, the system is configured to determine anestimate of a distance, along the thickness of the x-ray detectorsub-module, between the point of detection of the x-ray photon in thex-ray detector sub-module and the initial point of interaction based onthe estimate of charge diffusion,

-   -   wherein the system is configured to determine the estimate of        the initial point of interaction based on the point of detection        and the determined estimate of a distance along the thickness of        the detector sub-module.

As an example, the system may be configured to measure or estimate thewidth of the charge diffusion, and to determine the distance between thepoint of detection and the initial point of interaction based on themeasured width of the charge diffusion or distribution.

Optionally, the system is configured to determine an estimate of thepoint of interaction of the incident x-ray photon in at least one of thetwo directions (x, z) over which the detector elements are distributedon a main side of the x-ray detector sub-module or wafer.

In this case, the system may be configured to determine an estimate ofthe point of interaction of the incident x-ray photon in at least one ofthe two directions (x, z) over which the detector elements aredistributed on the main side based on information of a charge cloudprofile in one or both of the two directions (x, z) over which thedetector elements are distributed on the main side of the x-ray detectorsub-module or wafer.

For example, the system may be configured for determining the chargecloud profile, performing curve fitting and finding out where the curvehas its peak and identifying the peak as the point of interaction in aparticular direction.

Alternatively, or complementary, the system may be configured todetermine an estimate of the point of interaction of the incident x-rayphoton in at least one of the two directions (x, z) over which thedetector elements are distributed on the main side by identifying thepixel that has detected the highest charge as the point of interaction.

As previously indicated, the charge diffusion may be represented by acharge cloud, and the detector elements distributed over the x-raydetector sub-module or wafer on a main side may be designed and arrangedto provide an array of pixels, where the pixels are smaller than thecharge cloud to be resolved.

Optionally, at least one of the x-ray detector sub-modules comprises asemiconductor substrate or material comprising a plurality of activeintegrated pixels arranged in the semiconductor substrate.

As an example, at least part of the analog signal processing may beintegrated into the active integrated pixels.

By way of example, each x-ray detector sub-module has a number of depthsegments of detector elements in the direction of the incoming x-rays.

Preferably, the x-ray detector sub-modules may be arranged one after theother and/or arranged side-by-side in a configuration to form aneffective detector area or volume.

For example, an anti-scatter module may be integrated between at leastsome of the x-ray detector sub-modules.

Preferably, the system may be configured to determine an estimate ofcharge diffusion based on induced current caused by moving electron-holepairs originating from the Compton interaction or interaction throughphotoeffect, as detected by detector elements distributed over the x-raydetector sub-module or wafer. According to a third aspect, there isprovided an x-ray imaging system comprising a system according to thesecond aspect.

By way of example, the x-ray imaging system may be a Computed Tomography(CT) system.

In a particular example, the x-ray imaging system further comprises anassociated image processing device connected to the x-ray detectorsystem for performing the image reconstruction.

According to a fourth aspect, there is provided a corresponding computerprogram and computer-program product.

In particular, there is provided a computer program comprisinginstructions, which when executed by a processor, cause the processor toperform the method as described herein.

For example, there may also be provided a computer-program productcomprising a non-transitory computer-readable medium having storedthereon such a computer program.

FIG. 15 is a schematic diagram illustrating an example of a computerimplementation according to an embodiment. In this particular example,the system 200 comprises a processor 210 and a memory 220, the memorycomprising instructions executable by the processor, whereby theprocessor is operative to perform the steps and/or actions describedherein. The instructions are typically organized as a computer program225; 235, which may be preconfigured in the memory 220 or downloadedfrom an external memory device 230. Optionally, the system 200 comprisesan input/output interface 240 that may be interconnected to theprocessor(s) 210 and/or the memory 220 to enable input and/or output ofrelevant data such as input parameter(s) and/or resulting outputparameter(s).

In a particular example, the memory comprises such a set of instructionsexecutable by the processor, whereby the processor is operative todetermine an estimate or measure of charge diffusion and estimate theinitial point of interaction along the thickness of the detectorsub-module based on the determined estimate of charge diffusion.

The term ‘processor’ should be interpreted in a general sense as anysystem or device capable of executing program code or computer programinstructions to perform a particular processing, determining orcomputing task.

The processing circuitry including one or more processors is thusconfigured to perform, when executing the computer program, well-definedprocessing tasks such as those described herein.

The processing circuitry does not have to be dedicated to only executethe above-described steps, functions, procedure and/or blocks, but mayalso execute other tasks.

The proposed technology also provides a computer-program productcomprising a computer-readable medium 220; 230 having stored thereonsuch a computer program.

By way of example, the software or computer program 225; 235 may berealized as a computer program product, which is normally carried orstored on a computer-readable medium 220; 230, in particular anon-volatile medium. The computer-readable medium may include one ormore removable or non-removable memory devices including, but notlimited to a Read-Only Memory (ROM), a Random Access Memory (RAM), aCompact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray disc, aUniversal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storagedevice, a flash memory, a magnetic tape, or any other conventionalmemory device. The computer program may thus be loaded into theoperating memory of a computer or equivalent processing device forexecution by the processing circuitry thereof.

Method flows may be regarded as a computer action flows, when performedby one or more processors. A corresponding device, system and/orapparatus may be defined as a group of function modules, where each stepperformed by the processor corresponds to a function module. In thiscase, the function modules are implemented as a computer program runningon the processor. Hence, the device, system and/or apparatus mayalternatively be defined as a group of function modules, where thefunction modules are implemented as a computer program running on atleast one processor.

The computer program residing in memory may thus be organized asappropriate function modules configured to perform, when executed by theprocessor, at least part of the steps and/or tasks described herein.

Alternatively, it is possibly to realize the modules predominantly byhardware modules, or alternatively by hardware. The extent of softwareversus hardware is purely implementation selection.

The embodiments described above are merely given as examples, and itshould be understood that the proposed technology is not limitedthereto. It will be understood by those skilled in the art that variousmodifications, combinations and changes may be made to the embodimentswithout departing from the present scope as defined by the appendedclaims. In particular, different part solutions in the differentembodiments can be combined in other configurations, where technicallypossible.

1. A method for enabling estimation of an initial point of interactionof an x-ray photon in a photon-counting x-ray detector, which is basedon a number of x-ray detector sub-modules or wafers, each of whichcomprises detector elements, wherein the x-ray detector sub-modules areoriented in edge-on geometry with the edge directed towards an x-raysource, assuming the x-rays enter through the edge, wherein eachdetector sub-module or wafer has a thickness with two opposite sides ofdifferent potentials to enable charge drift towards the side, where thedetector elements, also referred to as pixels, are arranged, and whereinthe method comprises: determining an estimate of charge diffusionoriginating from a Compton interaction or an interaction throughphotoeffect related to the x-ray photon in a detector sub-module orwafer of the x-ray detector; and estimating the initial point ofinteraction along the thickness of the detector sub-module based on thedetermined estimate of charge diffusion.
 2. The method of claim 1,wherein each of the x-ray detector sub-modules comprises detectorelements distributed over the detector sub-module or wafer in twodirections including the direction of the incoming x-rays.
 3. The methodof claim 1, wherein the method is performed for determining, for each ofa number of incident x-ray photons and/or each of a number of x-raydetector sub-modules, a corresponding estimate of charge diffusion, andfor determining an estimate of the initial point of interaction of theincident x-ray photon in the respective x-ray detector sub-module. 4.The method of claim 1, wherein the estimate of charge diffusion isdetermined based on induced current caused by moving electron-hole pairsoriginating from the Compton interaction or interaction throughphotoeffect, as detected by detector elements distributed over the x-raydetector sub-module or wafer.
 5. The method of claim 1, wherein the stepof determining an estimate of charge diffusion comprises measuring orestimating the shape and/or width of the charge diffusion.
 6. The methodof claim 5, wherein the charge diffusion is represented by a chargecloud, and the estimate of charge diffusion is determined by measuringor estimating the shape and/or width of the charge cloud.
 7. The methodof claim 6, wherein the initial point of interaction of the incidentx-ray photon along the thickness of the detector sub-module is estimatedbased on the measured width of the cloud and the integrated charge ofthe cloud.
 8. The method of claim 1, wherein the step of estimating theinitial point of interaction along the thickness of the detectorsub-module comprises: determining an estimate of a distance, along thethickness of the x-ray detector sub-module, between the point ofdetection of the x-ray photon in the x-ray detector sub-module and theinitial point of interaction based on the estimate of charge diffusion,and determining the estimate of the initial point of interaction basedon the point of detection and the determined estimate of a distancealong the thickness of the detector sub-module.
 9. The method of claim8, wherein the width of the charge diffusion is measured or estimated,and the distance between the point of detection and the initial point ofinteraction is determined based on the measured width of the chargediffusion or distribution.
 10. The method of claim 1, wherein the methodfurther comprises determining an estimate of the point of interaction ofthe incident x-ray photon in at least one of the two directions (x, z)over which the detector elements are distributed on a main side of thex-ray detector sub-module or wafer.
 11. The method of claim 10, whereinthe step of determining an estimate of the point of interaction of theincident x-ray photon in at least one of the two directions (x, z) overwhich the detector elements are distributed on the main side isperformed based on information of a charge cloud profile in one or bothof the two directions (x, z) over which the detector elements aredistributed on the main side of the x-ray detector sub-module or wafer.12. The method of claim 11, wherein the method involves determining thecharge cloud profile, performing curve fitting and finding out where thecurve has its peak and identifying the peak as the point of interactionin a particular direction.
 13. The method of claim 10, wherein the stepof determining an estimate of the point of interaction of the incidentx-ray photon in at least one of the two directions (x, z) over which thedetector elements are distributed on the main side is performed byidentifying the pixel that has detected the highest charge as the pointof interaction.
 14. The method of claim 1, wherein the charge diffusionis represented by a charge cloud, and the detector elements distributedover the x-ray detector sub-module or wafer on a main side provide anarray of pixels, where the pixels are smaller than the charge cloud tobe resolved.
 15. A system for enabling estimation of an initial point ofinteraction of an x-ray photon in a photon-counting x-ray detector,which is based on a number of x-ray detector sub-modules or wafers, eachof which comprises detector elements, wherein the x-ray detectorsub-modules are oriented in edge-on geometry with the edge directedtowards an x-ray source, assuming the x-rays enter through the edge,wherein each detector sub-module or wafer has a thickness with twoopposite sides of different potentials to enable charge drift towardsthe side, where the detector elements, also referred to as pixels, arearranged, and wherein the system is configured to determine an estimateof charge diffusion originating from a Compton interaction or aninteraction through photoeffect related to the x-ray photon in adetector sub-module or wafer of the x-ray detector; and wherein thesystem is configured to estimate the initial point of interaction alongthe thickness of the detector sub-module based on the determinedestimate of charge diffusion.
 16. The system of claim 15, wherein eachof the x-ray detector sub-modules comprises detector elementsdistributed over the detector sub-module or wafer in two directionsincluding the direction of the incoming x-rays.
 17. The system of claim15, wherein the system is configured to determine, for each of a numberof incident x-ray photons and/or each of a number of x-ray detectorsub-modules, a corresponding estimate of charge diffusion, and todetermine an estimate of the initial point of interaction of theincident x-ray photon in the respective x-ray detector sub-module. 18.The system of claim 15, wherein the system is configured to determinethe estimate of charge diffusion by measuring or estimating the shapeand/or width of the charge diffusion.
 19. The system of claim 18,wherein the charge diffusion is represented by a charge cloud, and thesystem is configured to determine the estimate of charge diffusion bymeasuring or estimating the shape and/or width of the charge cloud. 20.The system of claim 19, wherein the system is configured to estimate theinitial point of interaction of the incident x-ray photon along thethickness of the detector sub-module based on the measured width of thecloud and the integrated charge of the cloud.
 21. The system of claim15, wherein the system is configured to determine an estimate of adistance, along the thickness of the x-ray detector sub-module, betweenthe point of detection of the x-ray photon in the x-ray detectorsub-module and the initial point of interaction based on the estimate ofcharge diffusion, wherein the system is configured to determine theestimate of the initial point of interaction based on the point ofdetection and the determined estimate of a distance along the thicknessof the detector sub-module.
 22. The system of claim 21, wherein thesystem is configured to measure or estimate the width of the chargediffusion, and to determine the distance between the point of detectionand the initial point of interaction based on the measured width of thecharge diffusion or distribution.
 23. The system of claim 15, whereinthe system is configured to determine an estimate of the point ofinteraction of the incident x-ray photon in at least one of the twodirections (x, z) over which the detector elements are distributed on amain side of the x-ray detector sub-module or wafer.
 24. The system ofclaim 23, wherein the system is configured to determine an estimate ofthe point of interaction of the incident x-ray photon in at least one ofthe two directions (x, z) over which the detector elements aredistributed on the main side based on information of a charge cloudprofile in one or both of the two directions (x, z) over which thedetector elements are distributed on the main side of the x-ray detectorsub-module or wafer.
 25. The system of claim 24, wherein the system isconfigured for determining the charge cloud profile, performing curvefitting and finding out where the curve has its peak and identifying thepeak as the point of interaction in a particular direction.
 26. Thesystem of claim 23, wherein the system is configured to determine anestimate of the point of interaction of the incident x-ray photon in atleast one of the two directions (x, z) over which the detector elementsare distributed on the main side by identifying the pixel that hasdetected the highest charge as the point of interaction.
 27. The systemof claim 15, wherein the charge diffusion is represented by a chargecloud, and the detector elements distributed over the x-ray detectorsub-module or wafer on a main side are designed and arranged to providean array of pixels, where the pixels are smaller than the charge cloudto be resolved.
 28. The system of claim 15, wherein at least one of thex-ray detector sub-modules comprises a semiconductor substrate ormaterial comprising a plurality of active integrated pixels arranged inthe semiconductor substrate.
 29. The system of claim 28, wherein atleast part of the analog signal processing is integrated into the activeintegrated pixels.
 30. The system of claim 15, wherein each x-raydetector sub-module has a number of depth segments of detector elementsin the direction of the incoming x-rays.
 31. The system of claim 15,wherein the x-ray detector sub-modules are arranged one after the otherand/or arranged side-by-side in a configuration to form an effectivedetector area or volume.
 32. The system of claim 31, wherein at leastpart of the detector elements have a longer extension in a direction ofthe incident X-rays than in a direction orthogonal to the direction ofthe incident X-rays, with a relation of at least 2:1.
 33. The system ofclaim 15, wherein the system comprises a processor and memory, thememory comprising instructions executable by the processor, whereby theprocessor is operative to determine an estimate or measure of chargediffusion and estimate the initial point of interaction along thethickness of the detector sub-module based on the determined estimate ofcharge diffusion.
 34. The system of claim 15, wherein the system isconfigured to determine the estimate of charge diffusion based oninduced current caused by moving electron-hole pairs originating fromthe Compton interaction or interaction through photoeffect, as detectedby detector elements distributed over the x-ray detector sub-module orwafer.
 35. An x-ray imaging system comprising a system according toclaim
 15. 36. The x-ray imaging system of claim 35, wherein the x-rayimaging system is a Computed Tomography (CT) system.
 37. Anon-transitory computer-readable medium having stored thereon a computerprogram comprising instructions, which when executed by a processor,cause the processor to perform the method of claim 1.